Using ARGB Quantized Palette: PNG 8BIT Doesn't Support Alpha

[mlaviexc]

M. Ansel,

For 32-bit ARGB, using your quantizer produces a palette, whose entries contain Alphas (4 ARGB bytes per panel entry).

The problem is that PNG 8-bit indexed images do not support Alpha in the palette. They only support specifying Transparent Color. Even Photoshop can't handle Alpha layer in 8-bit indexed PNG.

Is there a way to pass your code a 32-bit ARGB PNG, and for you to return an RGB-only palette, together with a transparent color?

TIA,
Lavie

[JeremyAnsel]

Hello,
You can make a copy of the initial data. Clear the alpha channel. Use WuColorQuantizer to reduce the image to 256 RGB colors. And then re-attach the transparency.

[mlaviexc]

Hello,

Thank you for the response.

According to the documentation I've read, PNG 8 Bit does not support alpha channel - only setting the transparent color's Palette Index. Am I correct? And since WuColorQuantizer only accepts RGB, there would be no indication of where the transparency should be.

Am I missing something?

Lavie